Okay, I feel we're getting somewhere. Thank you for your responses.
Yes. The class is determined by precedented usage. So I wonder how some people could consider up in She put the tent up as a preposition instead of a particle adverb, when there is such heavy and common usage. The same with all other phrasal verb particles, and with good as adverb in response to How are you? and with fast as adverb in drive fast and so on.
Just to be clear about your own position: Are you happy to accept up as adjective in Your computer is up? If so, is this then based on its precedented use as an adjective, ie., the weight of all the times it has ever been used as a modifier? That is, that what determines the word class is usage over form?
Regarding how we can know that good is an adjective:
I would dispute that better and best are forms of good. They both sound completely different and have separate etymologies so it's hard to accept that they are expressions of form of the root adjective good. My claim is that the only measure by which we class good as an adjective is its usage as a modifier.... from the fact that it has (admittedly highly irregular) comparative forms, and that it can be modified by an adverb.
Okay. I don't want to take this point any further at the moment because I'm not sufficiently convinced either way. I just wanted to know your position.The difference between good in your two examples is that in Only the good die young, good must imply people or, if we are talking about some other group of living things, that group. In the good of mankind, we understand good as an abstract concept. It is not synonymous with good cause (or any other noun).
To attempt to draw some conclusions, this is the main thrust of my argument so far:
1) The criteria we use when classing words as nouns/adjectives, etc, constitute a range of both formal and functional features. We know the class by what forms it can express and by how it can be used in context.
2) In some cases, there are only functional features which are available to determine word class. Sometimes the only way we can tell the class is by usage. Either a particular use in context or by a general precedented usage.
3) There are always functional features. Language does not exist outside of usage.
4) Therefore, function takes precedence over form as an indicator of word class. When the formal features of a word dictate a certain class, but the use dictates a different class, then usage takes precedence.
Please criticise and agree/disagree.